Auto-negotiation's role in plug-and-play

As engineers who maintain professional media networks, we may plug in hundreds or even thousands of Ethernet RJ-45 cables during our careers. In almost every case, the links just work. Understanding how we are able to interconnect 10Base-T, 100Base-T and GigE devices is worth knowing, especially when devices do not behave as expected.

Interoperability

In the early days of computer networks, a number of different cable mediums were tried, including RG-11 coax, RG-59 coax and Unshielded Twisted Pair (UTP). In the end, UTP and the RJ-45 connector won out over other alternatives. At the same time, Ethernet became the dominant networking technology. (There were other early networking technologies such as ARC-Net and Token Ring, but in almost all cases, these were dropped in favor of Ethernet.)

The first widely deployed Ethernet technology was 10Base-T, operating at 10 Mb/s. 10Base-T can operate either half-duplex (meaning that a Network Interface Card (NIC) can send or receive, but not at the same time), or in full-duplex (meaning that NIC cards can send and receive simultaneously). 10Base-T networks were deployed all over the world. But, even as 10Base-T was rising to become the dominant networking technology, designers came up with a faster network known as 100Base-T, capable of running at speeds up to 100Mb/s. And, of course, now we have GigE, or 1000Base-T, running at 1Gb/s. All of these standards support either half-duplex or full-duplex operation.

Today, plug a variety of computers, network switches and other networking devices into a wired Ethernet network, and they all pretty much just work. But, what makes this possible?